At Graphel Carbon Products, we know that you’re concerned about the latest industry trends and products worldwide. That’s why we have a blog about graphite and EDM solutions. Take a look through our articles down below to learn something new, then contact us with questions.
There are many different types of tooling, the most common being work holding tools. Work holding tools include jigs and fixtures; cutting tools for milling and grinding machines; dies for cold forming, forging and extrusion machines; and welding and inspection fixtures. In this month’s blog, we are going to look at the basics of grinding.
Grinding, or abrasive machining, is the process of removing metal in the form of minute chips by the action of irregularly shaped abrasive particles. These particles may be in bonded wheels, coated belts, or simply loose.
Grinding wheels are composed of thousands of small abrasive grains held together by a bonding material. Each abrasive grain is a cutting edge. As the grain passes over the work piece it cuts a small chip, leaving a smooth, accurate surface. As each abrasive grain becomes dull, it breaks away from the bonding material.
Types of abrasives
Two types of abrasives are used in grinding wheels: natural and manufactured. Except for diamonds, manufactured abrasives have almost entirely replaced natural abrasive materials. Even natural diamonds have been replaced in some instances by synthetic diamonds.
The manufactured abrasives most commonly used in grinding wheels are aluminum oxide, silicon carbide, cubic boron nitride, and diamond.
Types of bonds
Abrasive grains are held together in a grinding wheel by a bonding material. The bonding material does not cut during grinding operation. Its main function is to hold the grains together with varying degrees of strength. Standard grinding wheel bonds are vitrified, resinoid, silicate, shellac, rubber and metal.
Abrasive grain size
The size of an abrasive grain is important because it influences stock removal rate, chip clearance in the wheel and surface finish obtained.
Grinding wheel grade
The grade of a grinding wheel is a measure of the strength of the bonding material holding the individual grains in the wheel. It is used to indicate the relative hardness of a grinding wheel. Grade or hardness refers to the amount of bonding material used in the wheel, not to the hardness of the abrasive.
Grinding wheel structure
The structure of a grinding wheel refers to the relative spacing of the abrasive grains; it is the wheel’s density. There are fewer abrasive grains in an open-structure wheel than in a closed-structure wheel. A number from 1 to 15 designates the structure of a wheel. The higher the number, the more open the structure will be; and the lower the number, the denser the structure will be.
Wheel Balancing, dressing and truing
All grinding wheels are breakable, and some are extremely fragile. Great care should be taken in handling grinding wheels. New wheels should be closely inspected immediately after receipt to make sure they were not damaged during transit. Grinding wheels should also be inspected prior to being mounted on a machine.
Quality Control in manufacturing is a needed process that ensures customers receive products that are free from defects and meet their needs.
Our customers expect us to deliver quality products in a timely manner. Quality is critical to satisfying our customers and retaining their trust and loyalty. It influences our reputation and helps to control our overall costs.
Recently, Graphel Carbon Products held a Kaizen event to help improve our quality and our on-time performance. One of the main pillars of LEAN methodology, a Kaizen is a specific tool to improve quality. The objective of a Kaizen is to improve productivity, reduce waste, eliminate unnecessary work and humanize the workplace.
Our Kaizen was held over a period of 3 days and included members of our engineering, operations, quality and manufacturing departments. They discussed current methodologies, best practices as well as requirements of all the departments. “In any process there are 3 considerations, the way we plan it to take place, the way we think it takes place, and the way it actually takes place. “ The Kaizen is a great tool for merging the desired and needed attributes of all of these to streamline and enhance a process”, stated Karl Schmidt, Graphel’s Quality Manager.
The results of the Kaizen were very interesting. It was determined that we could utilize our Quality Coaches on the floor much more effectively by intermittently testing with mechanical inspection tools, and complete the final inspection with advanced technologies in our quality department. With the use of micrometers, calipers and gages, time can be freed up on the CMM in the Quality Department for the more complicated quality requirements.
Quality Coaches are a group of certified professionals dedicated to improving quality and productivity. Graphel Carbon Products has over 20 individuals on our production floor who have received additional training and perform some of the functions of quality staff. It was determined that by investing in training, we could reduce the hours in quality by 549, freeing up time and netting a 29% productivity improvement. By furthering the training of the quality coaches and operators, we freed up resources that could be utilized for other jobs, netting a overall productivity improvement of 58%.
By taking the complete responsibility for quality out of the quality department and bringing it to the production floor, our team was able to free up resources and streamline productions.
Quality is part of our culture at Graphel Carbon Products, and we believe it is everyone’s responsibility.
There are many different types of tooling, the most common being work holding tools. Work holding tools include jigs and fixtures; cutting tools for milling and grinding machines; dies for cold forming, forging and extrusion machines; and welding and inspection fixtures. In this month’s blog, we are going to look at turning.
Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helix toolpath by moving more or less linearly while the workpiece rotates. Turning is a form of machining, a material removal process, which is used to create rotational parts by cutting away unwanted material. The turning process requires a turning machine or lathe, workpiece, fixture, and cutting tool.
Turning is used to create rotational parts by cutting away unwanted material. The workpiece is a piece of pre-shaped material that is secured to the fixture, which itself is attached to the turning machine, and allowed to rotate at high speeds. The cutter is typically a single-point cutting tool that is also secured in the machine, although some operations make use of multi-point tools. The cutting tool feeds into the rotating workpiece and cuts away material in the form of small chips to create the desired shape.
Turning is used to produce rotational, typically axi-symmetric, parts that have many features. Features would include holes, grooves, threads, tapers, various diameter steps, and even contoured surfaces. Parts that are fabricated completely through turning often include components that are used in limited quantities, such as custom designed shafts and fasteners.
Turning is also commonly used as a secondary process to add or refine features on parts that were manufactured using a different process. Due to the high tolerances and surface finishes that turning can offer, it is ideal for adding precision rotational features to a part whose basic shape has already been formed.
The quality of a finished part depends on the precision and characteristics of the tooling. Its’ properties, the speed and accuracy with which it can be produced and the repeatability of manufacture in high volume production runs, all depend on the precision and characteristics of the tooling. So, for the best parts, tooling needs to be designed and engineered to the highest quality.
As the cut progresses through the work metal a cavity starts to form. The deeper this cavity becomes, the harder it is for fresh dielectric fluid to get into the cavity to remove debris and quench the work piece and electrode. In order to get smooth, even flow of dielectric through the gap, flushing becomes an essential part of the EDM process.
Good flushing allows the work piece particles and eroded electrode particles to be removed from the gap. Flushing also allows fresh dielectric into the gap. Both are necessary to maintain stable cutting and to prevent arcing.
It is the volume of oil moving through the gap that performs particle removal. Turbulence in the tank would indicate not enough volume and too much pressure. The ideal pressure is usually between 3 to 5 psi. Flushing at higher pressure may actually prevent the flow of particles out of the gap and the dielectric renewal in the gap. High pressure also tends to wear the electrode.
The balance of volume and pressure is important. Roughing operations where the gap is large would require high volume and low pressure for good oil flow. Finishing operations where the gap is smaller may necessitate higher pressure to improve the oil flow.
The three basic types of flushing are pressure, suction, and external. The choice of flushing method may be limited by the application. The electrode shape and size may prohibit through the electrode flushing.
This is the most common method of flushing. Pressure flushing forces the dielectric fluid through holes in the electrode into the gap between electrode and workpiece. Fluid and particles flow up the sides of the cavity. Pressure flushing through the electrode helps cool the electrode.
This is the opposite of pressure flushing. Fluid and particles are pulled out of the gap through holes in the electrode or workpiece. This method reduces secondary discharge and tapered walls.
Nozzles or tubes may be used to direct streams of fluid into the gap opening. Fluid and particles are pushed out the opposite side. This is the least desirable method of flushing. Poor flushing conditions can trap particles which may cause DC arcing and pitting.
EDM machining is considered by most to be a thermal removal process. The most convincing support for this claim is the removal of material from the electrodes by melting and/or vaporization by a thermal process, along with pressure dynamics established in the spark-gap. The spark gap generates an electrical force on the surface of the electrode that removes material.
Electric discharge machining (EDM), sometimes referred to as spark machining, spark eroding, burning, die sinking or wire erosion is a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks). Material is removed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool-electrode, or simply the ‘tool’ or ‘electrode’, while the other is called the workpiece-electrode, or ‘workpiece’.
Electrode gap (spark gap) is the distance between the electrode and the part during the process of EDM. Electro-mechanical or hydraulic systems are used to respond to average gap voltage. To obtain good performance and gap stability a suitable gap should be maintained.
The EDM process is based on the thermoelectric energy. This energy is created between a workpiece and an electrode submerged in a dielectric fluid with the passage of electric current. The workpiece and the electrode are separated by a specific small gap called spark gap. Pulsed arc discharges occur in this gap filled with an insulating medium, preferably a dielectric liquid like hydrocarbon oil or de-ionized (de-mineralized) water. In this process there is no direct contact between the electrode and the workpiece thus eliminating mechanical stresses, chatter and vibration problems during machining.
As an electrode moves toward the workpiece the spark gap is reduced so that the applied voltage is high enough to ionize the dielectric fluid. Short duration discharges are generated in a liquid dielectric gap, which separates electrode and workpiece. The material is removed from tool and workpiece with the erosive effect of the electrical discharges. The dielectric fluid serves the purpose to concentrate the discharge energy into a channel of very small cross sectional areas. It also cools the two electrodes, and flushes away the products of machining from the gap.
Engineering materials having higher thermal conductivity and melting points are used as a tool material for EDM process of machining. Copper, graphite, copper-tungsten, silver-tungsten, copper graphite and brass are used as a tool material (electrode) in EDM. They all have good wear characteristics, better conductivity, and better sparking conditions for machining. Tungsten resists wear better than copper. The factors that affect selection of electrode material include metal removal rate, wear resistance, desired surface finish, cost of electrode material manufacture and material and characteristics of work material to be machined.
Choosing Dielectric Fluid for Sinker EDM Applications
Choosing the correct dielectric fluid for your EDM application is not always as straightforward as it might seem. Many criteria need to be taken into account. Some are obvious, such as degree of metal removal and electrode wear, while others are much more subtle.
Dielectric fluid is a material whose main purpose is to prevent or rapidly quench electric discharges. Dielectric liquids are used as electrical insulators in high voltage applications to provide electrical insulation, suppress corona and arcing, and to serve as a coolant.
A good liquid dielectric should have high dielectric strength, high thermal stability and chemical inertness against the construction materials used, non-flammability and low toxicity, good heat transfer properties, and low cost. Liquid dielectrics are self-healing; when an electric breakdown occurs, the discharge channel does not leave a permanent conductive trace in the fluid.
Sinker EDM machines typically use hydrocarbon oil for their dielectric fluid, into which both the workpiece and spark are immersed. In contrast, wire EDM machines normally use deionized water, into which only the sparking area is immersed. Whether oil-based or water-based, the dielectric fluid used in EDM machines serves three critical functions:
• Controlling the spacing of the sparking gap between the electrode and workpiece
• Cooling the heated material to form the EDM chips
• Removing EDM chips from the sparking area
Although they’re considerably smaller than those produced in milling or turning processes, EDM does produce chips. These tiny, hollow spheroids are composed of material from both the electrode as well as the workpiece. Just like any chip, they need to be removed from the cutting zone, which is accomplished by flowing the dielectric fluid through the sparking gap.
As the dielectric fluid breaks down—whether as the result of age or contamination—the risk of unstable discharge increases. Control electronics can compensate to a certain extent, but the only real solution is to continually pump clean dielectric fluid through the cutting zone to flush it. The more conductive particles in the fluid, the more difficult it is for the machine to maintain stable electrical thresholds inside the sparking gap.
Because the lifespan of dielectric fluid depends on a host of factors, such as its type and the efficiency and quality of your EDM fluid filters, it has no definitive expiry date. As a rule of thumb, however, if you’re using an oil-based fluid and it’s over five years old, it should probably be replaced. You can also perform sight and smell comparisons between used and new fluids, but the best way to determine whether your dielectric fluid needs to be replaced is with a refractometer.
Choosing the right dielectric fluid for your EDM application is not always as straightforward as it might seem. Many criteria need to be taken into account. Some are obvious, such as degree of metal removal and electrode wear, while others are much more subtle.
SINKER EDM BASIC TERMINOLOGY – PART II – FLUSHING
MAR 2018 Blog
It’s been said the 3 most critical things about a good sinker EDM burn condition are flushing, flushing, and flushing. This is still true today but modern machines have built in technologies that assist with flushing even when the operator doesn’t think about flushing. More on these in a bit.
Flushing is a critical part of the EDM process, as it removes contaminated fluid and eroded particles, and replaces them with clean, temperature controlled fluid. This removes contamination from the spark gap that could develop into undesirable conditions like slow, unstable burns, pitting of the work piece, and possibly destructive DC arcing. Flushing also helps cool the work piece by replacing warmed EDM fluid with chilled fluid. Spark temperatures, even though extremely small, can be in the 15,0000 to 22,0000 F range.
There are a few basic types of flushing. The first discussed here will be Injection Flushing, called such because cooled dielectric fluid is injected through the electrode into the spark gap. This is the most efficient type of flushing for many jobs as it basically pressurizes the spark gap around the electrode to keep a continuous flow of fluid going from the bottom of the cavity up the sides, and out the top. This removes eroded particles and keeps the work area cool. Injection flushing is particularly effective for large, deep, and/or complex shaped burns.
Things to be careful of when using injection flushing:
• Over burn at the top of the cavity due to secondary erosion. This is caused by excess conductive debris being washed out of the burn making the sparks longer.
• A post created where the flushing hole is. This can become unstable as it gets taller and moves around under flushing pressure inside the flushing hole. This can cause sporadic short circuit problems with the burn.
• The area around the flushing holes can become distorted and worn unevenly due to the flow and pressure of the flushing.
Another basic flushing style is Suction Flushing. With suction flushing, a vacuum line is attached to the bottom of a cavity or detail being burned, and the fluid and debris flows downward to the suction point. Suction flushing is very effective in burns with materials that develop smaller debris particles, like carbides. Many times suction flushing will be used together with a part holding fixture, and it is not uncommon to see suction and injection flushing used together on some applications.The last basic flushing style covered here will be Lateral Flushing. Also referred to as Cross Flushing or Sweep Flushing, this form of flushing uses external nozzles to cause of flow of dielectric fluid across the part being EDM’d. Lateral flushing is, in general, the least effective form of flushing discussed here. IF the burn is very shallow and not too big of an area, lateral flushing is effective. It is also effective in setting up flow directions with some burns. Flushing nozzles placed in opposite corners of a cavity can cause a swirl effect around the entire electrode. As with injection flushing, a drawback is deformation of the electrode by the flow and impact of the fluid eroding the electrode in some areas.
Back to those built in technologies I mentioned earlier. Many modern EDM machines have several technologies that assist with or compensate for poor flushing situations.
• High speed axis drives
Can accelerate/decelerate in multiple-G range
Causes a stirring of fluid and debris in the spark gap
Fresh cool fluid sucked in upon retraction as dirty warmed fluid is forced out
• Rotating C-Axis spindles
Extremely effective in drilling deep holes
Rotation of electrode causes swirling of fluid, keeping debris from gathering in one place
• Highly adaptive advanced control systems
Monitors spark activity in real time
Makes changes to spark parameters to prevent a bad condition in the spark gap
• Programmable flushing systems built into machines
No matter how its achieved, flushing is still a critical part of the EDM process. For more information refer to your machine’s user manual or technical support.
RAM EDM, otherwise known as “Sinker EDM” or “Plunge EDM” has been around for over fifty years. Though widely used, there are some who are unfamiliar with certain EDM terminology.
KEY FACTORS OF SINKER EDM
Since Sinker EDM uses electrical energy to remove material, it stands to reason that the make-up of the material will have a direct impact on the rate of removal. This being the case there are three critical things to keep in mind.
• Since machining with Sinker EDM or Plunge EDM occurs by using electrical energy the electrical conductivity plays a key role in how fast the Spark Gap can ionize allowing a spark to occur.
• Materials that are less electrically conductive will machine slower, such as carbides and PCD.
• Pulse energy (heat) is conducted away from the surface of the part with flushing and temperature controlled dielectric fluid. Some of this heat will dissipate into the work piece. Certain materials, like beryllium copper alloys used in molds, are designed to dissipate heat quickly. This will slow down the metal removal rate of the EDM process.
THE EDM PULSE
When we see a flash of lightning we are witnessing EDM on a grand scale. The lightning bolt is electrical energy flowing between an electrode (cloud) and a (grounded) workpiece in a natural kind of EDM phenomena. This discharge is the same as what occurs in an EDM machine.
• An EDM Pulse is highly controllable and there are parameters to determine the size and intensity of the spark.
• On-Time is the length of time the spark is turned on. This determines the depth the spark can travel into the workpiece. On-Time will have a direct impact on final part size and surface finish.
• Off-Time is the period of time between the end of one spark and the ignition of the next. Off-Time allows for efficient chip removal and cooling in the spark gap.
• The combination of the EDM Pulse On and the Pulse Off is one cycle. It has been said that the EDM process can develop as many as 250,000 cycles per second, but only one spark at a time.
• Amperage is the electrical power of the spark. The higher the amperage, the more aggressive the spark, and the deeper the spark can go into the metal. Like On-Time, Amperage will have a direct impact on final part size and surface finish.
The combination of ON TIME and AMPERAGE are the two parameters that will determine spark length, or overburn. Changing the ON TIME and/or AMPERAGE during a burn will change the size and finish of the final result. All other machine parameters will affect MMR and/or electrode wear, but not spark length. To better understand your machine’s parameters, consult your owner’s manual or machine builder.
A key area for improvement in EDM operations is the reduction of consumables. New technologies, machine settings and improved material grade limit ram or sinker EDM electrode wear to 0.1% while maintaining productive machining speeds. For wire EDM, new low-consumption technologies reduce the biggest expense—the wire itself—by as much as 50 percent.
With all EDM machines you experience the benefits of designing and cutting complex shapes and tapered holes with hard metals. You can depend that the machine has the capacity to cut exactly what you want.
Sinker EDM machines use an electrode and workpiece submerged in liquids such as oil or dielectric water. A power supply is connected to the electrode and generates electrical potential between both of the parts, producing a breakdown to form a plasma channel and spark jumps. The sparks initiated by the power supply often times strike one another.
In the sinker EDM process, wear on the electrode starts as soon as the erosion process begins. As metal is burned away on the workpiece, the electrode gradually experiences wear and loses it’s fine details and is dimensionally changed. Minimizing electrode wear is not only critical to reducing costs and lead times, but also improving part accuracy.
From a general sinker EDM perspective, quality graphite electrode materials provide the most productive machining speed. The wear rate of a graphite electrode depends largely on the size of the detail, the electrode reduction amount, and the power settings used. But the grade of the graphite is a contributing factor. Using the correct grade of graphite will limit wear and rate of erosion.
Wire electrical discharge machining uses a single string of thin metal wire to cut thick metals for precise incisions and splits. Similar to Sinker EDM, Wire EDM uses an electrode and spark to cut metal. Using a spark erosion technique, Wire EDM machining submerges the part being cut in deionized water and the wire acts as the electrode, creating a spark that roughs or skims the part into the desired shape without the wire ever coming in contact with the part.
The price of a wire EDM machine is minimal when compared to the cost of the wire over the life expectancy of the machine. Excessive wire consumption on a wire electrical discharge machine is costly. Technology that allows slower unspooling speeds without compromising results appears to be the answer. Wire is the single highest expense in operating a wire EDM. With even the least expensive EDM wire running $5 to $6 per pound, investing in low-wire consumption EDM machines appears to be the answer.
A key area for improvement in EDM operations is the reduction of consumables. For Sinker EDM users, consider using better grades of quality materials to reduce cost. For Wire EDM users, consider investing in new technology with machine settings that reduce the amount of wire used.